In the recent years, a first communication system (first RAT (Radio Access Technology) that does not support CS (Circuit Switching) calls is known. The first communication system, for example, is an LTE (Long Term Evolution)-compatible communication system.
CS FALL BACK (hereinafter, referred to as CSFB) is known as a technique of controlling the incoming and outgoing of a CS call of a mobile communication terminal (UE) in such a first communication system (For example, Non Patent Literatures 1 to 5).
In the CSFB, the transition to a second communication system (second RAT) that supports CS calls is instructed to the mobile communication terminal (UE). A technique of performing an Inter-Rat handover of a PS (Packet-switching) call, which is set up in the first communication system, to the second communication system, (hereinafter, referred to as a PS handover) is known as a method of transition to the second communication system. In the PS handover, after a PS call is set up in the second communication system, a CS call is set up in the second communication system.
Specifically, during the PS handover, when the mobile communication terminal is in an idle state in the first communication system, after the setup of the PS call between the mobile communication terminal and the first communication system, the PS call set up in the first communication system is carried over to the second communication system.
Alternatively, during the PS handover, when the mobile communication terminal is in the connected state in the first communication system, a PS call set up between the mobile communication terminal and the first communication system is carried over to the second communication system.
Furthermore, the PS handover is considered to be advantageous due to the fact that particularly when PS communication is being performed in the first communication system, the mobile communication terminal can be connected to the second communication system without disrupting the PS communication, by carrying over the PS calls to the second communication system.